In related art, there is an electronic device in which a plurality of electronic instruments or shelf-mount electronic units (hereinafter referred to as mount units) having the same height and width are placed in a storage-dedicated rack (which may be called a shelf and hereinafter referred to as a case). In this electronic device, the mount units are used as being horizontally placed in a longitudinal direction of the case or being longitudinally placed in a horizontal direction.
In an air-cooled electronic device, to keep a normal cool state of each mount unit, cooling air is suitably supplied to each mount unit with balance. However, when a mount unit is extracted from the electronic device for repair or replacement, a large amount of cooling air flows into a space from which the mount unit has been extracted (which is a space to accommodate a mount unit and is called a slot), and the cooling balance among the remaining mount units is destroyed.
For this reason, when a mount unit is extracted from the electronic device, some measures are taken such as increasing the number of revolutions of a cooling fan or mounting a shielding plate on the case for closing the opening of a slot after a mount unit is extracted from the case.    [Patent document] Japanese Unexamined Utility Model Registration Application Publication No. 5-21492
However, if the number of revolutions of the cooling fan is increased to enhance supply capability of cooling air to increase the volume of air when an out-of-order mount unit is extracted from the electronic device, noise and electric power are increased. Moreover, the cooling fan is expected to have blowing capability appropriate to a maximum volume of air, and therefore is disadvantageously large in size and high in price with the use of a high-performance motor capable of high speed revolutions.
Furthermore, when a shielding plate is mounted on the case, the case itself has a mount structure for mounting the shielding plate, and therefore a limitation on the structure of the case tend to occur.